I believe he was a French king who ruled shortly before the Renaissance, but I'm probably wrong.

The story goes like this:

A knowledgeable courtier reported to the king that there was in his kingdom a man who could tell the future: not quite an oracle, nor a wizard: merely a God-fearing fellow who had this special gift and whose fame was growing by the minute.

The King demanded that the seer be brought to him.

Shortly before the man's arrival at the palace, the King gathered his guards and told them to wait outside the chamber in which the interview was to take place and, upon the seer's exit, grab him and cut his throat.

As the guards stood waiting outside, the seer entered the chamber. After a brief exchange of greetings, the King asked:

"When am I going to die?"

The seer was an insightful man. He quickly realized what was to follow and, upon some reflection, replied:

"The week following my death, Sire."

After composing himself as best he could, the King expressed his gratitude to the seer. When the seer was on his way out, the King followed him closely, and, the moment the door opened, gestured desperately to the guards from behind the seer's back that they should stand the hell down. The guards got the point, and did.

1 Answer
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This anecdote is from Walter Scott's novel Quentin Durward. The king is Louis XI.
The guy who predicts the future is Martius (Marti) Galeotti, Italian astrologist.
The guard who is supposed to kill him at the exit of king's chamber is Oliver, barber and servant of Louis.

I think Scott made it up. But possibly he read this is some historical source.
Martius Galeotti (1442-1494) was a real person (see Wikipedia), as well as Oliver and Louis XI.